Traditional AC-DC power switching power supplies consist of an AC-DC power conversion stage and a DC-DC power conversion for output voltage regulation. Their output filter usually consists of an inductor and a capacitor forming an output voltage filter. The schematic of a traditional AC-DC converter with electric isolation can be shown in FIG. 1. It consists of two stages: an AC-DC power stage and a DC-DC power conversion stage.
For electrically isolated output voltage, which is required in many power supplies, electrical isolation is usually achieved with the use of an isolation transformer. The DC-DC power converter usually consists of an inverter bridge (fed by a DC voltage from a front AC-DC power stage), an isolation transformer, a rectifier and an output filter comprising an inductor and a capacitor. Generally, voltage feedback from the “secondary side” of the transformer is required and the control action for the output voltage regulation is carried out by the inverter on the “primary side” of the transformer. It is important to note that this traditional approach requires the output filter Lout and Cout to filter the switching voltage ripple.
In order to reduce the conduction loss in the diode rectifier, synchronous rectification can be used. Synchronous rectification has been utilized in switched mode power supply technology. The replacement of diodes with power MOSFETs with low on-state resistance enables synchronous rectifiers to have less conduction loss than diodes. This has been adopted in switched mode power supply for computer products which have Central Processing Units (CPUs) running at low voltage and high current conduction (e.g. 3.3 V DC at 100 A). In conventional switched mode power supply applications, closed-loop output voltage control is an essential feature because the output voltage of a power supply must be controlled within a tight tolerance.
In existing synchronous rectification technology, the output voltage regulation is primarily controlled from the primary side of the system. FIG. 2 shows a typical schematic of a DC-DC power converter using a synchronous rectifier based on National Semiconductor design document titled “Synchronous Rectification in High-Performance Power Converter Design” authored by Robert Selders Jr., and available at the website: http://www.national.com/appinfo/power/files/national_power_designer112.pdf.
In this traditional DC-DC converter with an isolated diode-based rectifier, the output voltage is controlled by the driving circuit on the primary side. Secondary feedback, via isolated means, is used to control the switching action in the primary circuit in order to regulate the output DC voltage.
The diodes in FIG. 2 can be replaced with power MOSFETs having low on-state resistance as shown in FIG. 3. Similar to the circuit in FIG. 2, the output voltage control is carried on the primary circuit with secondary feedback provided through isolated means. In addition, an output inductor Lout is needed.
In principle, the secondary gate controller can be eliminated if a self-driven gate drive design is adopted. A self-driven synchronous rectifier takes advantage of the polarities of the induced voltages in the secondary winding. Such a self-driven synchronous rectifier is shown in FIG. 4 and a corresponding control scheme is shown in FIG. 5. Despite the fact that the secondary gate drives can be eliminated, the output voltage regulation is still controlled by the primary circuit.
In both cases, the synchronous rectifiers, regardless of using diodes or MOSFETs, provide the AC-DC rectification only. The output voltage regulation is controlled by the switching action in the primary circuit.
The problems of the traditional approach when employed in a wireless transfer system are summarized as follows:
(a) Two power stages, i.e. AC-DC and DC-DC without transformer isolation, are needed. This increases the cost and size of the circuit and is not attractive for embedding into a portable device such as a mobile phone, particularly, one with a slim design.
(b) Output voltage regulation is controlled by the inverter switching action on the “primary side” of the isolation transformer. This means an isolated feedback mechanism is required, which leads to increased cost.
(c) An output inductor Lout is needed. This increases power loss and reduces energy efficiency of the secondary circuit, leading to: thermal problems in a portable device, which typically has no ventilation; safety problems in the battery due to a high temperature rise; and a reduction in overall system efficiency.
If output voltage regulation is needed without control from the primary circuit of the transformer, one solution is to use a DC-DC converter with voltage control as shown in FIG. 6. The AC voltage induced in the secondary winding is first rectified, and then the DC-DC converter will turn the rectified voltage into a regulated DC voltage. However, this approach:
(a) is a 2-stage method;
(b) requires a DC-DC converter, such as the ones described above; and
(c) requires an output inductor.
These three factors increase the cost and size of the secondary module and reduce the overall energy efficiency of the system.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.